A usual subscriber line technology (DSL) offers fast data transfer on existing copper-based telephone lines. In DSL, broad-band data signals are transmitted on significantly higher frequencies than traditional narrow-band telephone signals. Since the narrow-band telephone signals and the broad-band data signals are both transmitted over the same subscriber line, splitter devices are provided for splitting and recombining the two types of signals at both ends of the subscriber line, i.e. at the central office or switching center, and at the end terminals at the subscriber location. There are various types of DSLs that have evolved over the last years such as ADSL, HDSL, MDSL, SDSL and VDSL. Multitone modulation is the basis of the DMT version of ADSL as well as some multi-carrier versions of VDSL. This type of modulation is also called orthogonal frequency division multiplexing (OFDM). In discrete multitone modulation (DMT), a given frequency range for data transmission is resolved into a number of narrow frequency bands for use as individual data links. In ADSL, data transmission occurs roughly between 20 kHz and 0.1 MHz.
In order to transmit the xDSL data signals over the telephone line which consists of a pair of copper wires, the central office must be provided with line drivers. The line driver compensates for the attenuation of the telephone line and has to comply with the PSD mask requirement of the DSL standard. The line driver amplifies the line-coded xDSL signal so that it is received downstream at the subscriber location with sufficient signal intensity. Similarly, the line drivers are provided at the subscriber locations for transmitting xDSL data upstream to the central office.
The basic component of each line driver is a power amplifier for amplifying the xDSL signal which is to be transmitted over the telephone line.
Conventional line drivers include linear class-B and AB amplifiers. However, the driving transistors in a class-AB amplifier are biased to operate in their linear region so that they are always in an on-state and draw quiescent current. This results in an inefficient power dissipation.
Accordingly, it has been proposed to employ class-D amplifiers in xDSL line drivers to improve the power efficiency. The class-D amplifier according to the state of the art comprises a self-oscillating loop for generating a switching frequency and a preamplifier which receives an input signal from a signal source. The self-oscillating loop contains a comparator that converts the analog input signal to a digital output signal. The preamplifier and the comparator create a variable duty cycle square wave signal. As a consequence, a pulse train is created wherein the duty cycle is proportional to the level of the input signal. This pulse width modulated signal is coupled to the gates of two complementary output transistors. The source drain paths of the two copper transistors are connected in series between a supply voltage VDD and ground GND. In effect, the pulse width modulated signal with a duty cycle proportional to the input signal level turns complementary output transistors on and off with a switching frequency which is much greater than the frequency of the input signal. Hence, power is sufficiently delivered from the power supply to the load.
Line drivers employing class-D power amplifiers achieve a higher power efficiency than conventional line drivers. The so-called switched mode line drivers are based upon a self-oscillating circuit core.
FIG. 1 shows a block diagram of a conventional broad-band line driver having a self-oscillating core. The self-oscillating core oscillates at a switching frequency of e.g. 10 MHz. The self-oscillating core switches the output stage with a switching frequency. A demodulation filter is provided at the output of the self-oscillating core for removing switching residuals from the output signal spectrum. Analog feedback is supplied outside the demodulation filter and fed back to the input of the self-oscillating core in order to define the gain of the amplifier and to adapt the termination impedance to the output of said line driver. At the input terminals of the self-oscillating core, the input signal generated by a signal source is superimposed on the feedback signals.
The analog line driver as shown in FIG. 2 according to the state of the art receives analog signals and outputs analog signals. However, the analog driver is built around a switched core where the switching frequency is dependent on the stability properties of the internal switched loop.
FIG. 2 shows a switched mode line driver such as in FIG. 1 in more detail. FIG. 2 shows an ADSL-line-interface-circuit connected to a CODEC circuit. The ADSL-line-interface-circuit can be used in the central office, DSLAM, DLC and MSAP applications. The ADSL interface circuit receives an analog input signal from a digital analog converter DAC of the CODEC via capacitors. The transmit input interface is a differential voltage interface. The ADSL-line-interface-circuit is connected to the output of the DAC via AC-coupling capacitors.
The ADSL-line-interface-circuit comprises one self-oscillating loop circuit. The oscillating loop circuit includes a switched output driver which switches with a switching frequency fs of the oscillating loop circuit. The input of the switched output driver contains a comparator that converts the analog input signal to a digital signal. The output of the switching output driver is fed back via a low-pass filter LPF1, LPF2 to adders which are connected to the inputs of the switched output driver. The output of switching output driver is connected to a demodulation filter. The output of the demodulation filter is fed back to the input of the oscillating core and applied to an analog adaption circuit for adapting the output impedance of the ADSL-line-interface-circuit to the impedance of the subscriber line.
The ADSL-line-interface-circuit comprises one self-oscillating loop circuit wherein the self-oscillating loop circuit has a loop signal which oscillates with a switching frequency fs. Both low-pass filters LPF are analog RC low-pass filters. The switching frequency fs of the oscillating loop circuit depends on the capacitance of the capacitors provided in both low-pass filters LPF1, LPF2.
The drawback of the ADSL-line-interface-circuit as shown in FIG. 2 is that the capacitance of the capacitors of the RC-low-pass filters LPF1, LPF2 is not adjustable. The switching frequency fs of the oscillating loop circuit depends on the capacitance of the low-pass filters. With increasing switching frequency fs, the current consumption of the ADSL-line-interface-circuit is increased. After production of the integrated ADSL-line-interface-circuit, the capacitance provided in the low-pass filters LPF1, LPF2 vary within certain limits so that the switching frequency fs of the self-oscillating loops is in many cases too high. As a consequence, the current consumption of the ADSL-line-interface-circuit according to the state of the art as shown in FIG. 2 varies in a broad range and is often too high.